This invention relates to the field of induced dynamic conducting interfaces. More particularly, this invention relates to laminar flow induced dynamic conducting interfaces for use in micro-fluidic batteries, fuel cells, and photoelectric cells.
A key component in many electrochemical cells is a semi-permeable membrane or salt bridge. One of the primary functions of these components is to physically isolate solutions or solids having different chemical potentials. For example, fuel cells generally contain a semi-permeable membrane (e.g., a polymer electrolyte membrane or PEM) that physically isolates the anode and cathode regions while allowing ions (e.g., hydrogen ions) to pass through the membrane. Unlike the ions, however, electrons generated at the anode cannot pass through this membrane, but instead travel around it by means of an external circuit. Typically, semi-permeable membranes are polymeric in nature and have finite life cycles due to their inherent chemical and thermal instabilities. Moreover, such membranes typically exhibit relatively poor mechanical properties at high temperatures and pressures, which seriously limits their range of use.
Fuel cell technology shows great promise as an alternative energy source for numerous applications. Several types of fuel cells have been constructed, including: polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz, the entire contents of which are incorporated herein by reference, except that in the event of any inconsistent disclosure or definition from the present application, the disclosure or definition herein shall be deemed to prevail.
Although all fuel cells operate under similar principles, the physical components, chemistries, and operating temperatures of the cells vary greatly. For example, operating temperatures can vary from room temperature to about 1000° C. In mobile applications (for example, vehicular and/or portable microelectronic power sources), a fast-starting, low weight, and low cost fuel cell capable of high power density is required. To date, polymer electrolyte fuel cells (PEFCs) have been the system of choice for such applications because of their low operating temperatures (e.g., 60–120° C.), and inherent ability for fast start-ups.
FIG. 1 shows a cross-sectional schematic illustration of a polymer electrolyte fuel cell 2. PEFC 2 includes a high surface area anode 4 that acts as a conductor, an anode catalyst 6 (typically platinum alloy), a high surface area cathode 8 that acts as a conductor, a cathode catalyst 10 (typically platinum), and a polymer electrolyte membrane (PEM) 12 that serves as a solid electrolyte for the cell. The PEM 12 physically separates anode 4 and cathode 8. Fuel in the gas and/or liquid phase (typically hydrogen or an alcohol) is brought over the anode catalyst 6 where it is oxidized to produce protons and electrons in the case of hydrogen fuel, and protons, electrons, and carbon dioxide in the case of an alcohol fuel. The electrons flow through an external circuit 16 to the cathode 8 where air, oxygen, or an aqueous oxidant (e.g., peroxide) is being constantly fed. Protons produced at the anode 4 selectively diffuse through PEM 12 to cathode 8, where oxygen is reduced in the presence of protons and electrons at cathode catalyst 10 to produce water.
The PEM used in conventional PEFCs is typically composed of a perfluorinated polymer with sulphonic acid pendant groups, such as the material sold under the tradename NAFION by DuPont (Fayetteville, N.C.) (see: Fuel Cell Handbook, Fifth Edition by J. Hirschenhofer, D. Stauffer, R. Engleman, and M. Kleft, 2000, Department of Energy—FETL, Morgantown, W. Va.; and L. Carrette, K. A. Friedrich, and U. Stimming in Fuel Cells, 2001, 1(1), 5). The PEM serves as catalyst support material, proton conductive layer, and physical barrier to limit mixing between the fuel and oxidant streams. Mixing of the two feeds would result in direct electron transfer and loss of efficiency since a mixed potential and/or thermal energy is generated as opposed to the desired electrical energy.
Operating the cells at low temperature does not always prove advantageous. For example, carbon monoxide (CO), which may be present as an impurity in the fuel or as the incomplete oxidation product of an alcohol, binds strongly to and “poisons” the platinum catalyst at temperatures below about 150° C. Therefore, CO levels in the fuel stream must be kept low or removed, or the fuel must be completely oxidized to carbon dioxide at the anode. Strategies have been employed either to remove the impurities (e.g., by an additional purification step) or to create CO-tolerant electrodes (e.g., platinum alloys). In view of the difficulties in safely storing and transporting hydrogen gas, the lower energy density per volume of hydrogen gas as compared to liquid-phase fuels, and the technological advances that have occurred in preparing CO-tolerant anodes, liquid fuels have become the phase of choice for mobile power sources.
Numerous liquid fuels are available. Notwithstanding, methanol has emerged as being of particular importance for use in fuel cell applications. FIG. 2 shows a cross-sectional schematic illustration of a direct methanol fuel cell (DMFC) 18. The electrochemical half reactions for a DMFC are as follows in acidic conditions:

As shown in FIG. 2, the cell utilizes methanol fuel directly, and does not require a preliminary reformation step. DMFCs are of increasing interest for producing electrical energy in mobile power (low energy) applications. However, at present, several fundamental limitations have impeded the development and commercialization of DMFCs.
One of the major problems associated with DMFCs is that the semi-permeable membrane used to separate the fuel feed (i.e., methanol) from the oxidant feed (i.e., oxygen) is typically a polymer electrolyte membrane (PEM) of the type developed for use with gaseous hydrogen fuel feeds. These PEMs, in general, are not fully impermeable to methanol. As a result, an undesirable occurrence known as “methanol crossover” takes place, whereby methanol travels from the anode to the cathode through the membrane. In addition to being an inherent waste of fuel, methanol crossover also causes depolarization losses (mixed potential) at the cathode and, in general, leads to decreased cell performance.
Therefore, in order to fully realize the promising potential of DMFCs as commercially viable portable power sources, the problem of methanol crossover must be addressed. Moreover, other improvements are also needed including: increased cell efficiency, reduced manufacturing costs, increased cell lifetime, and reduced cell size/weight. In spite of massive research efforts, these problems persist and continue to inhibit the commercialization and development of DMFC technology.
A considerable amount of research has already been directed at solving the aforementioned problem of methanol crossover. Solutions have typically centered on attempts to increase the rate of methanol consumption at the anode, and attempts to decrease the rate of methanol diffusion to the cathode (see: A. Heinzel, and V. M. Barragan in J. Power Sources, 1999, 84, 70, and references therein). Strategies for increasing the rate of methanol consumption at the anode have included increasing catalyst loading (i.e., providing a larger surface area), increasing catalyst activity (i.e., increasing efficiency), and raising operating pressure and/or temperature. Strategies for decreasing the rate of methanol diffusion to the cathode have included decreasing methanol concentrations, fabricating thicker NAFION membranes, synthesizing new proton conducting materials having low permeability to methanol, lowering cell operating temperature, and fabricating methanol tolerant cathodes. However, to date, there remain pressing needs in DMFC technology for significantly lowered fabrication costs, increased efficiency, extended cell lifetimes, and appreciably reduced cell sizes/weights.